This invention relates to hoppers and, more particularly, to gravity motivated batch type hoppers for handling bulk material, for example, hoppers used in charging coke ovens.
Hoppers which are charged with batch quantities of a material are referred to as batch type hoppers. If the hopper depends upon gravity to motivate contained material through an opening in its lower portion, it is referred to as gravity motivated or operated hopper. Hoppers are commonly employed to handle bulk materials that are granular solids of varying particulate sizes, as, for example, crushed coal or cement, flour, clay and soil.
During the discharge of gravity motivated hoppers, a serious problem, referred to as arching or bridging, can occur. Arching occurs when a portion of the material in the hopper becomes consolidated and forms a stable arch which extends between the walls of the hopper and prevents or retards the discharge of the remaining material from the hopper. Consolidation is used in reference to the action of such material. Consolidation is influenced by many factors. One factor is the average particulate size of the material. Additional factors are moisture content of the material and material compaction. Compaction refers to the conditioning which results from successive layers of material being charged, layer upon layer, into the hopper.
Many approaches have been tried in order to eliminate arching, for example, selection of hopper construction material to minimize friction, design of a hopper to include steep walls and use of vibrators. Each of these approaches has met with limited success.
The size of the discharge opening can be carefully selected to take into consideration the flow characteristics of the material to be handled by a hopper as the flow characteristics of the particular granular material can be measured and used to predict the minimum size of a bottom discharge opening, which will prevent the formulation of a stable arch. This minimum size is equal to the critical arching dimension, or arch cross-sectional flow area. If the opening is equal to the crital arching dimension, a stable arch will not form in the hopper above it. However, if the opening is smaller than the critical arch dimension, a stable arch can form. Frequently the size of the discharge opening is required to be smaller than the critical arching dimension. The size of the discharge opening often fixed by external factors such that the size of the charging opening with which the hopper must correspond, or dimensions of a receptacle which is to be charged by the hopper. Various types of internally mounted static flow promotion devices have been tried. These devices have taken the form of flat plates, cones and cylinders, to name a few, and are generally mounted inside the hopper above the discharge opening. Such static devices help prevent arching by minimizing consolidation of the material during and after charging. Material falling into the empty hopper arrives at the bottom after being slowed down by striking the device, therefore, the device continues to absorb part of kinetic energy of the falling material, as well as support part of the material after charging is complete.
While the static devices have helped to prevent arching problems, they also create problems. Arching of material tends to occur in the area between such devices and the hopper wall. Thus, while preventing arching in one area of the hopper, the static device may actually create a structure which can lead to arching in another location. Also, the static devices affect the downward flow of material directly above the device. This affect can have the result of slowing the speed of discharge of the hopper affecting system performance for which the hopper can be an integral part.
Gravity operated batch type hoppers are widely used as a part of the charging machine to charge coke ovens. Here, the arching problems associated with these hoppers are not only present but are aggravated by conditions found in coke oven operations. For instance, arching can result from compaction due to charging of crushed coal from the storage bin into the hopper carried on a larry car charging machine. In addition, increased consolidation takes place due to vibration when the larry car moves from the charging bin to a charging hole in the coke oven furnace. As the amount of moisture present in the coal increases, the danger of arching increases considerably due to the presence of coal dust which, affected by moisture, forms an adhesive agent.
Arching can cause insufficient material to be charged into the coke oven, resulting in lost production and less uniform coke quantity. Finally, prolonged hopper discharge time due to arching can result in increased amount of pollutant gases escaping from furnace interior.
The present invention alleviates the arching problem without inhibiting the hopper discharge time.